In depth

Learn more about our three-port converter in this Rectiverter Wiki

The Rectiverter is a technology created by Eltek. The Rectiverter is a three port module with an AC input port, an AC output port, and a bidirectional DC port. It is intended for the use of building a modular AC and DC UPS. Up to 12 modules can be configured in parallel preferably distributed over three phases.

Rectiverter Functionality

The Rectiverter contains a rectifier function and an inverter function. The Rectiverter can be used as a rectifier and inverter at the same time. The available power at the AC input port will be available and distributed to the AC and DC output ports. In the case of loss of AC, the Rectiverter will continue to work as an inverter without any interruption of AC output power (seamless transition). Both DC and AC load will be supported from the battery.

It can also be used as a telecom rectifier only, or as an inverter only.  Just two of the three ports are used in these configurations. These building blocks are ideally suited for building an isolated AC to AC UPS.

Rectiverter features

Line conditioning

An essential feature of the Rectiverter is the ability to stabilize and filter the AC mains voltage, stay connected and draw energy also from a disturbed AC grid. This is possible due the active control of the AC output voltage.

Working in AC/AC mode of operation, the AC output port exhibits a power line conditioning function by supplying a filtered and regulated stable AC voltage to the load.

AC grid voltage THD up to 10% is effectively eliminated.

Transients on the grid such as ring waves caused by AC load switching are effectively filtered.

Loss of mains AC voltage source - seamless switch over to DC source

The Rectiverter will continue to support the load connected to the AC output port with stabilized AC voltage as long as a charged battery is connected to the DC port. The seamless switching of energy source provides an uninterrupted AC supply in case of loss of AC mains voltage. The AC output will never experience any transient or interruption. From the prioritized AC load point of view, the change of energy source is seamless.

Seamless reconnection to AC

The Rectiverter will reconnect to AC mains and start to draw power from the AC input port once mains voltage has recovered to normal after an abnormal condition.

This action contains a series of steps, including determining that AC mains is within the nominal range, synchronizing the AC output voltage to be in phase with AC input and physically connect the input AC relay.

The seamless reconnection is completed within 10 to 30 seconds.

Surge OV protections

There can be extreme overvoltages in the AC distribution if lightning hits the grid. Power supplies such as the Rectiverter are tested for this kind of surge pulses. The rise time and fall time of this overvoltage is measured in microseconds (tr/tf Ô 1.2/50 µs). The input protection relay cannot respond quickly enough for this type of short overvoltage pulse. Instead, built in MOV protection devices will clamp the voltage and fast overcurrent protection turns off semiconductor switches, so that the over current pulse can be safely absorbed by internal electrolytic capacitors.

It must be noted that a proper lightning protection must be installed at the entrance of the building or equivalent ZONE 1 boundary. The Rectiverter´s SPDs are intented for ZONE 3 boundary, Class III protection devices, so a Zone 2, Class II SPD is desirable for maximum reliability.

Backfeed protection

The Rectiverter has a unique electronic backfeed protection which has passed an extensive test program in order to comply with international UPS standards. The UL standard prescribes the following requirement regarding so called backfeed protection. 

"A UPS shall be provided with backfeed protection to prevent involving a risk of electric shock from being present on its input ac terminals during interruption of the input power." 

In other words, if the operator disconnects the AC input source from the system, he or she shall be assured that there is no hazardous AC voltage being fed back from the AC output. If the operator accidently shorts the AC output to the AC input terminals of the Rectiverter, the Rectiverters turns off, and no hazardous voltage will be present at the input.

Modes of operation

Inverter operation

The Rectiverter works as a pure inverter if there is no voltage present at AC-input port.

Inverter mode of operation means that power is transferred from the DC port to the AC output port. In order for the Rectiverter to operate as an inverter, the DC port must be connected to an energy source, such as a rectifier and/or a battery, capable of providing DC voltage to the Rectiverter.

Rectifier operation

The Rectiverter works as a rectifier if AC voltage is present at the AC input port and AC output port is disabled/not connected. Energy is transferred from AC input port to the DC port. The load on the DC port can be either a battery that needs to be charged and or other DC loads. A typical load can be -48 V telecom equipment.

Rectiverter operation

Rectiverter operation means that power is transferred from AC input port to both AC output and DC port simultaneously. The Rectiverter can supply both an AC load and DC load in this mode of operation.

Mains voltage

Mains electricity differ between regions in the world. The Rectiverter is factory pre-set to either nominal 115VAC or 230VAC input and output voltage. Note that the Rectiverter has wide AC input voltage and frequency ranges which make it possible to use the Rectiverter in a majority of countries/regions of the world. 

The Rectiverter is programmed to accept either 50Hz or 60Hz mains frequency. The adaptive frequency option enables the Rectiverter to automatically determine the mains frequency, and synchronize. A tolerance of +/- 3 Hz is programmed from factory. The tolerance range can be set by the user from the controller. 

ROCOF, Rate of Change of Frequency

Distributed generators may not always provide stable and precise frequency. Diesel generators or wind generators often produce slowly varying and unstable frequency. The Rectiverter is able to synchronize and draw energy from an AC source with varying frequency. The maximum rate of change of frequency, ROCOF, should be < 0.2Hz/second in order to comply with commonly accepted practice.

A(50Hz) <-> B(60Hz) mains switching

Especially in marine applications where the AC mains is produced by diesel generators, there might be a need for switching from 60Hz AC generated by diesel generators to 50 Hz commercial grid. The adaptive frequency options can be used as follows.

The 60Hz AC is switched off, the Rectiverter goes to inverter mode, the 50Hz voltage is thereafter connected. The Rectiverter will then synchronize to the 50Hz grid and reconnects.  The whole process takes about 45 seconds. The AC load is supported from the batteries during this time. AC output voltage is unaffected, except for the change of frequency.

AC input voltage range

The AC input voltage range is divided into four different regions:

  • High voltage disconnect - the total output power is supported from batteries
  • The normal voltage range for which the full output power is sourced from AC input port
  • Low voltage range, with linear derating down to shut off level, the AC output power is prioritized
  • Low voltage shut down disconnect - the total output power is supported from batteries
Short mains interruptions and hold - up

There is 5 ms hold-up capability for uninterrupted operation of the Rectiverter. Shorter interuptions (< 5ms) of AC mains will not affect the operation of the Rectiverter. The AC output will not be affected, nor the DC output. Longer AC mains interruptions will cause the Rectiverter to go into inverter mode of operation.  If AC mains returns within a few line cycles, the reconnection is done within seconds, due to the synchronization is still in place. 

A 10ms hold-up capability is available when running in rectifier mode of operation.

Power factor

It is of great concern to transfer power with as high Power Factor (PF) as possible in order to utilize the grid transmission network as much as possible. Reactive power will just provide losses in the network and will not reach the end users as real useful active power.

Loads should ideally not introduce reactive power to the network. Power Factor Correction (PFC) is therefore mandatory for larger electronic loads.

The Rectiverter has a PFC stage at the AC input port. Harmonic distortion of load currents at the AC-output port caused by non-linear loads will therefore not be reflected to the grid.  The Rectiverter will always act as a linear, resistive load to the grid when operating at maximum load.

AC mains overvoltage conditions

The Rectiverter can withstand over-voltages on the input AC port for long time. Although not intended to work >275VAC it can withstand continuous overvoltages substantially higher than 275VAC. This is possible due to the built in relay which disconnects the AC input power stage at an overvoltage condition.  The AC output will be maintained during an AC mains overvoltage condition as the Rectiverter runs in inverter mode supplying energy to the load from the batteries.

AC output port

VA rating

The VA rating can be seen as a measure of over current capability and especially the ability to feed non-linear loads.  The VA rating for the AC-output port of the Rectiverter is based on measurements of RMS voltage and RMS current at maximum active power supplied to a standardized non-linear load. A typical standardized load includes a diode bridge and a capacitor. 

The diode bridge will cause peak rectification and thus introduce higher frequency components in the load current, so called harmonics. The harmonics will not contribute to active power, but affect the measurement of the RMS current. 

Non-linear load and crest factor

The Rectiverter has a peak current set to a maximum value to provide safe operation of the semiconductors in the AC-output stage.

The maximum RMS current is set to a value to limit the active power to its maximum value at nominal AC output voltage.

The crest factor for the Rectiverter is calculated as:

C rated = I peak / I RMS_max

A typical non-linear load would be a diode bridge connected to a filter capacitor providing a filtered DC voltage to a DC load.

Connecting this type of diode bridge/capacitor/load to a low ohmic AC source would result in so called "peak rectification". The name illustrates the charging of the filter capacitor at the peaks of the AC voltage. The capacitor will be charged almost instantaneously with very high peak current spikes  as a result.

The Rectiverter handles peak rectifier loads very well. The load peak current will be limited by the maximum current limitation of the AC output stage. The AC output load current will therefore look like flat peaks, square pulse train. The conduction angle will vary with the amount of active load. 

Capacitive load

The Rectiverter can supply large capacitive (and inductive) loads.

The load can be purely capacitive, cos∅=0.

The capacitive current (reactive current) can exceed the active (resistive) load current even at maximum load, as can be seen in the following discussion.

Capacitive load means that there will be a reactive current in addition to the real active load current. This reactive current will increase the total RMS current delivered by the AC-output stage. The Rectiverter has a maximum RMS current limitation set by the need for protecting the AC output fuse in continuous operation.  The max RMS current for continuous operation is therefore set to approximately 150% of the nominal active RMS current limitation,IP_max.

Imax_RMS=1.5*IP_max

We have the following relationship for total RMS current, IRMS, active, IP, and reactive, IQ, currents:

I RMS = IP2 + IQ2

So, at maximum active load there is an absolute maximum allowed reactive load current: 

-> IQ_max ≈ 1.1*IP_max

Overload on AC port

The Rectiverter can support a temporary 150% overload on AC output for 15 seconds. This feature is primarily intended to facilitate startup of various loads. The converter goes into current limit if the overload is applied for longer time then 15 seconds. The current limit exhibits "fold back" characteristics as the current limit is reduced after 15 seconds.

Quick trip - fuse clearance

Quick trip has big impact on system design. Enough short circuit current will available for tripping circuit breaker with a minimum number of units without the need for an extra parallel Static Transfer Switch.

The so-called Quick Trip function is intended for tripping an MCB in a short circuit condition. The Rectiverter will supply a 6 x Inom RMS current for 20 ms when a short circuit is detected. 

It works like this: when the AC-output voltage is below approximately 30 V the Quick Trip Pulse (QTP) is released and AC output current limits are elevated to 6 x Inom during the first 20 ms after the short is applied. From this it follows that two modules working in parallel are able to supply 12 x Inom and so on. 

The quick trip pulse is available in AC/AC mode, and DC/AC mode of operation, meaning that it is a property of the AC output port only. Regardless if the energy is supplied from the AC input port or the DC port, the quick trip is available.

Systems Considerations

Mains configurations and Protective Earth considerations

Because the Neutral (N) wire is fed straight through the Rectiverter, the mains feed should have an unbroken path from N to Protective Earth (PE), i.e. the N should not be broken in any distribution point from the source to the Rectiverter system. If the feed is broken the N must be bonded to PE in the system with a link of the same rating as the incoming feed.

In single phase systems without N, both incoming conductors must be disconnected if the incoming feed is shut off. This can be achieved by using a contactor which is controlled by the incoming feed. The Rectiverter system cannot be used in D-connected 3-phase configurations.

Three phase operation

Although the Rectiverter is a single phase unit, it can also be used in systems for Y connected three – phase power transmission. Rectiverters can be connected between any of the three phase voltages and a common neutral connection. The neutral needs to be connected to the AC-inputs for proper current sharing and voltage control.

A three-phase Rectiverter system may feed a true three-phase load without a neutral connection.

The position of a HW strap in each power shelf determines the proper phase connection. Should wrong phase be connected, a “wrong phase alarm” will be issued, and the modules with “wrong phase” will not start up, and an alarm LED will turn RED on the module front.

Static bypass switch

Centralized static bypass switches (also called Static Transfer Switch, STS) are commonly used in UPS systems for bypassing the UPS, connecting the load directly to the grid. The switch has a great overload capability and will supply required short circuit currents for tripping fuses for fault clearings. A disadvantage of a centralized static bypass switch is the need for a separate compartment/cabinet for the switch itself. Other disadvantages are the fact that you need to design for full system power from start, and the fact that the centralized switch will introduce a single point of failure in the system.

The Rectiverter module contains an AC/AC connection which eliminates the need for a separate centralized static bypass switch. In addition, the AC output port is designed for supplying an overcurrent pulse for tripping MCBs and clear short circuit currents. The advantages of the bypass switch are maintained while the single point of failure is eliminated.

Automatic Transfer Switch (ATS)

The ATS is used for switching between different AC sources. The two AC sources can either be two separate grid connections, or a grid connection and a diesel generator.

Physical implementation

Power shelf

The power shelf has four slots for housing one Rectiverter each. The power shelf contains back planes for AC and DC connectors, CAN and sync bus. The shelf can be mounted in a 1 U, 19 inch cabinet slot.

The power shelf can be combined with a distribution and control unit. The distribution unit contains a controller, I/O units and DIN rails for holding AC and DC distribution MCBs, battery connections and battery MCBs, as well as screw terminal sockets for AC input and DC bus connections.

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